A rail car cannot be called a semi-monocoque structure. Same as as tractor trailer .. there is a substantial I-beam under the 'box' structure that routes the tensile & majority of torsion. The 'box' structure at most just deals with the 'air' loads. A monochrome structure handles a fore-said loads as well as bending & internal pressurization. Remove the I-beam .. & the rail car become a mass of sheet metal collapsed on the ground.

Examples of large opening .. There are 2 types of arrangements. 'Plug' and 'Integral'. Plug arrangement - A structure is built around the opening that is capable of transferring the shear loads around the opening. The door is just a 'plug' to either/or separate the interior from the environment and/or contain the internal pressurization. Ie: Plug .. Opening frame so designed that internal forces push the door into a frame which is slightly smaller than the area of the door - thus plugging the opening. The loss of this 'door' is usually not detrimental to the overall integrity of the structure.
- An 'integral' door contains mechanisms that bind it to the surround structure of the opening. By means of hooks & latches it re-establishes a continuous load path across the opening. Most modern jetliners are of this type. A loss of the 'door' would significantly endanger the integrity of the structure.